Single-panel, color LCD systems are commonly driven by signals generated in response to an optical scan, produced by the successive scrolling of differently colored stripes, usually red, green and blue, over the pixels making up the panel, and an electrical scan, representing addresses of the rows of pixels with the signal corresponding to the color of the light impinging on the row. The optical scan is often generated by a set of three scanning prisms, although rotating color wheels and other means have also been employed. Although the prisms are intended to rotate at a constant angular velocity, the stripes produced on the surface of the panel do not necessarily move with constant linear velocity. However, the electrical address scan signal is generated, and moves through the rows of the panel in a linear manner. This means that the electrical scan and the optical scan, i.e., the leading edge of the color stripe for which data is provided by the electrical scan, may not be separated by a time interval sufficient to permit the pixels to switch in intensity, thereby producing inter-color mixing artifacts.
Typically, a given row of the panel is addressed with data corresponding to one color, followed by scanning the panel with the stripe of that color. The same row, with a first, fixed offset, is then addressed with data corresponding to the second color, followed by scanning the panel with the stripe of the second color. The same row, with a second, fixed offset, is then addressed with data corresponding to the third color, followed by scanning the panel with the stripe of the third color. The next row is then addressed with data corresponding to the first color, and so on. In order to minimize the aforementioned variations in the time period between the electrical and optical scans, thereby reducing the color errors resulting from inter-color mixing artifacts, the two fixed offsets correspond to the size of the optical stripes (in row distances) as if they were to be measured at the center of the panel. A manual adjustment of the relative rotation phases of the three prisms is then made to minimize the color errors visually with a set of red, green and blue test patterns. However, due to the potential mismatch of the electrical and optical scans, i.e., variations in the time period between the electrical address signal at a particular location on the panel and impingement of the leading edge of the color stripe at that location, the system remains susceptible to color errors.
The present invention is directed to overcoming one or more of the problems or disadvantages associated with the relevant technology.
The present invention provides a method of establishing a delay of at least a minimum duration between the electrical addressing of the panel locations with the data for the next color stripe to be scrolled across the panel and the leading edge of that color stripe impinging upon the addressed location; preferred apparatus for implementing the method is also disclosed and forms a part of the invention. The fixed delay is selected to allow sufficient time for pixel switching from one color value to another, and will vary depending upon the frame rates of the system and the liquid crystal response time. For purposes of the present discussion, the minimum duration of the delay is 2.5 ms. The fixed delay is achieved by three arrays of photosensors, each with a filter rendering it sensitive to one of the color stripes which are scrolled across the panel. The photosensor arrays are integrated on the display panel itself, e.g., in three, laterally adjacent, vertical areas along the right side of the panel.
As each horizontal color stripe is scrolled vertically down the panel surface, the-sensor array for each color will generate signals indicating the row location of the leading and/or lagging edges of the respective color stripe at any instant in time. The signals from each sensor array are fed to a control circuit which can adjust-the location of the next electrically addressed row for the respective color. The control circuit is adapted to determine the best choice of which row to address next and with which color data information based on the relative speeds of the color stripes as they are scrolled across the panel. That is, rather than following a fixed sequence of row addressing for each color (i.e., row N is addressed with red data, then green data, then blue data, row N+1 is addressed with red, then green, then blue, row N+2 is addressed with red, etc.) the control circuit may instruct the addressing in a different order, responsive to changes in the relative speeds of scanning of the color stripes, to ensure at least a minimum time delay between addressing and scanning. By “synchronizing” the addressing and scanning functions, the invention creates a better color rendition of the displayed image free of inter-color mixing artifacts.